Predicting the resistance of basil entries to downy mildew based on their genetics, pathogen race, growth stage, and environmental conditions

A model predicting the level of resistance of basil to downy mildew was developed. The model integrates plant age, genetic background, sporulation, disease intensity, pathogen races, and environmental data at an early stage of disease. These results can be used to select and develop new basil cultivars and accelerate the time needed in breeding for basil downy mildew resistance. Basil downy mildew (BDM) caused by the oomycete Peronospora belbahrii emerged as a global threat, rapidly becoming the most devastating disease of sweet basil (Ocimum basilicum) and other Ocimum spp. worldwide. Despite advancements in understanding its biology and epidemiology, and the availability of approved fungicides and management strategies, BDM remains economically destructive and an ongoing risk to basil production worldwide. Recently, the development and introduction of resistant cultivars have emerged as crucial tools in BDM management and the emergence of new BDM races creates new challenges to controlling this disease. The present study aimed to provide growers and breeders with insights into the survival capabilities of resistant basil cultivars under varying genetic backgrounds, pathogen races, growth stages, and various environmental conditions. Through a series of lab and field experiments, we evaluated the response of multiple resistant sources and their lineages to various isolates of P. belbahrii across different locations, using multiple indices to assess their resistance. Entries carrying the R genes Pb1/Pb2 exhibited complete resistance across all races, growth stages, and environmental conditions. Those harboring the R-gene Pb2 showed similar resistance levels, with minor variability due to growth stage. Responses of Pb1 plants varied with pathogen race, displaying full resistance to race 0 at all growth stages but displaying susceptibility to race 1. Plant cultivars possessing MRI resistance genes and their recombinant inbred lines (RIL's) exhibited variable responses to pathogen attacks, ranging from high tolerance to complete susceptibility. Some MRI RIL's showed high resistance similar to Pb2 entries. Pb0 cultivars and 'Eleonora' (unknown background) were susceptible to all races and growth stages in all experiments. Comprehensive analysis across all genetic backgrounds revealed a significant correlation (R = 0.73) between disease intensity (D.I) at the seedling stage under controlled conditions and D.I in adult plants under field conditions. Principal Component Analysis (PCA) across six experiments indicated that the primary components influencing disease outcomes were the accession, race, and growth stage, explaining 65%, 22%, and 7% of the variability, respectively. A prediction model based on the statistical parameters residual (%) and root-mean-square error (RMSE) demonstrated strong predictability, particularly regarding pathogen sporulation and daily disease development rates. The model predicted resistance probabilities with R² values of 0.81, 0.91, and 0.93 at the second, third, and final disease score readings, respectively, significantly earlier (~ 14-21 days post-infection) than traditional assessments (~ 42 days). These findings demonstrate that resistance in basil entries against current pathogen races can be effectively assessed within weeks of disease onset, facilitating more timely and informed management decisions for growers and providing an important tool for plant breeders in search of improved BDM resistance.